(ôs'kär hĕrt'vĭkh) , 1849–1922, German embryologist. He studied medicine with Haeckel and Gegenbaur. In 1875 he established the fact that fertilization consists of the union of the nuclei of a male and a female sex cell. He studied the germ-layer theory (introducing the term coelom) and malformations of vertebrate embryos.